CUI Affected Units / Areas in LNG Equipment:

Corrosion Under Insulation (CUI) occurs when water enters external insulation through holes or gaps in the insulation covering, or when moisture in the air condenses on the metal surface below the insulation (known as sweating). The water tends to collect at low spots, insulation support rings and other external appendages, or simply beneath the insulation. If an adequate protective coating is not present, significant external corrosion can occur. The often random and localized nature of the attack, and the fact that it is hidden under the insulation, makes CUI difficult to detect.

Critical Factors

• Location Issues 

Common areas of concern in process units are higher moisture areas such as those areas down-wind from cooling towers, near steam vents, deluge systems, acid vapors, or near supplemental cooling with water spray.

• Design Issues

CUI can be found on equipment with damaged insulation, vapor barriers, weather proofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges.

• Equipment designed with insulation support rings welded directly to the vessel wall (no standoff ); particularly around ladder and platform clips, and lifting lugs, nozzles and stiffener rings. 

Prevention / Mitigation

• Since the majority of construction materials used in plants is susceptible to CUI degradation, mitigation is best achieved by using appropriate paints/coatings and maintaining the insulation/sealing/vapor barriers to prevent moisture ingress. 
• High quality coatings, properly applied, can provide long term protection.
• Careful selection of insulating materials is important. Closed-cell foam glass materials will hold less water against the vessel/pipe wall than mineral wool and potentially be less corrosive. 
• Low chloride insulation should be used on 300 Series SS to minimize the potential for pitting and chloride SCC. 
• CUI can be found on equipment with damaged insulation, vapor barriers, weatherproofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges. 
• Equipment designed with insulation support rings welded directly to the vessel wall (no standoff ); particularly around ladder and platform clips, and li fting lugs, nozzles and stiffener rings.

Source:http://www.adgas.com/En/Publication/Corrosion%20Booklet.pdf